896 THE ORGANS OF SENSE. 



ligament of the lens, thus establishing a connection between the choroid and inner 

 tunic of the eye. They are arranged in a circle, and form a sort of plaited frill 

 behind the iris round the margin of the lens. They vary between sixty and 

 eighty in number, lie side by siae, ana may be divided into large and small ; the 

 latter, consisting of about one-third of the entire number, are situated in the 

 spaces between the former, but without regular alternation. The larger processes 

 are each about one-tenth of an inch in length, and are attached by their periphery 

 to the Ciliary muscle, and are continuous with the layers of the choroid : the 

 opposite margin is free, and rests upon the circumference of the lens. Their 

 anterior surface is turned toward the back of the iris, with the circumference of 

 which they are continuous. The posterior surface is closely connected with the 

 suspensory ligament of the lens. 



Structure. The ciliary processes are similar in structure to the choroid, but 

 the vessels are larger, and have chiefly a longitudinal direction. They are covered 



Anterior ciliary artery. 



Short ciliary arteries. ' 



vLnierior ciliary artery. 



FIG. 530. The arteries of the choroid and iris. The sclerotic has been mostly removed. (Enlarged.) 



on their inner surface with a layer of black pigment-cells continuous with the cells 

 of the pigmentary layer of the retina, and in their stroma are also other, stellate, 

 pigment-cells, which, however, are not so numerous as in the choroid itself, and 

 toward the free extremities of the folds are devoid of pigment. 



The Iris (iris, a rainbow) has received its name from its various colors in dif- 

 ferent individuals. It is a thin, circular-shaped, contractile curtain, suspended 

 in the aqueous humor behind the cornea and in front of the lens, being perforated 

 a little to the nasal side of its centre by a, circular aperture, the pupil, for the 

 transmission of light. By its circumference it is intimately connected with the 

 choroid ; externally to this is the Ciliary muscle, by which it is connected to the 

 sclerotic and cornea ; its inner edge forms the margin of the pupil ; its surfaces 

 are flattened, and look forward and backward, the anterior surface toward the 

 cornea, the posterior toward the ciliary processes and lens. The circumference 

 of the iris is connected to the cornea by a reticular structure denominated the 

 ligamentum pectinatum iridis. The anterior surface of the iris is variously 

 colored in different individuals, and marked by lines which converge toward the 

 pupil. The posterior surface is of a deep purple tint, from being covered by dark 

 pigment ; it is hence named uvea, from its resemblance in color to a ripe grape. 



Structure. The iris is composed of the following structures : 



1. In front is a layer of polyhedral cells on a delicate hyaline basement 

 membrane. This layer is continuous with the epithelial layer of the membrane of 

 Descemet, and in men with dark-colored irides the cells contain pigment-granules. 



2. Stroma. The stroma consists of fibres and cells. The former are made up 



